Cathode for electron discharge devices and method of making the same



Patented Aug. 8, 1933 CATHODE FOR 1 92 0 6 f ,7 ELECTRON DISCHARGE DE-VICES AND METHOD THE 7 :SAME r Leslie HerbertBedford, Aldwych,London,.Eng

" land, assignor to. Western Electric Company,

Incorporated, New York, N. Y., a Corporation of New York No Drawing.Application August 13, 1929, Serial 7 N 0. 385,697, and in Great BritainNovember 16,

, 2 Claims. (01. 250-275) This invention relates to thermionicfilamentsor cathodes for use in electron discharge devices, valves or, the like.i a

The object of the invention is to providesuch a cathode or filamentwhich has good emission and long life.

According to one feature of thejinvention a filament or cathode ismanufactured so that it has an active surface which comprises one ormore alkaline earth metals, and one or'more'me tallic oxides.

According to another feature the alkaline cathode and are alloyedtogether. Preferably said surface comprises metallic barium andba- 1rium oxide.

, ide or strontium oxide or a'mixture.

Other features of the invention will be disclosed in the followingdescription with reference' to a preferred method or process of pro-'ducingthe cathode or filament of the present inventioni In thermionicfilaments of the oxide type the thermionic activity is attributed bysome to the alkaline earth oxide and by others to a supposed film ofalkaline earth metal. Ihave conducted experiments which lead to thehypothesis that there exists a complex of. barium metal and barium oxidewhich has a much higher activity than either constituent alone and thesame is probably true of the other alkaline earth metals and of the rareearth metals. The present in-- vention is believed to provide a cathodehaving 'an active surface comprising a high percentage 7 of thiscomplex. Thermionic filaments made in In order to obtain a cathodeaccording to the procedure may be iol-- invention the following lowe d-A core or filament is provided with a coating.

of oxide, which may for instance be barium ox- The en-. velopecontaining the filament and other-electrodes is then evacuated andduring the evacua tion a carefully determined amount 'of alkaline earthor rare earth metal is applied or added to the filament. This is moreeasily performed and controlled bypassing an ionizing current' throughlow pressure vapour of alkaline earth or ried to the cathodeby apotential gradient. The

following method is preferably employed to effect the introduction ofthe barium source into the envelope. A mixture of barium oxide, barium.peroxide, and metallic aluminum is heated, whereupon at a' certaintemperature (depending principally upon proportions) a reaction rium andaluminum oxide. readilydistilled from the mixture. The application tothe thermionic device may take place at any stage, that is to say eitherbefore or after the performance of the thermit reaction, or after earthor rare earth metals, and oneor more metallic oxides, form the activesurface of the,

thedistillation of the barium; Measures are desirable to preventoxidation or hydrating of the source while it is being applied, forpreference,

to the plate of the valve. Saturated hydrocarbons carefully dried overvsodium have proved suitable as protection, but anyother form may beused, such as another metal. It is convenient to provide a small pocketin the plate (anode) of the valve which may contain the barium sourceand in orderto control the egress of the barium vapour therefrom acovering plate may be used to provide only a restricted outlet from saidpocket. Any other means of reducing the vapour pressure of the bariummay-be used instead of the above. r

. During the operation of the device, the space lytic effect, toestablish a concentration of oxide at the cathode surface, with acorresponding concentration of alkaline earth or rare earth metal atthecore; counteracting this is the normal difiusion effect tending toannul all concentration gradients. The relative concentration of oxideat the surface is also increased by the evaporation of the metals whichgenerally have a higher vapour pressure than the oxides.

current from the cathode tends, by an electro: 1

For these reasons the addition of alkaline'earth.

or, rare earth metals is carried further than is required by theproportion of the complex by a certain amount which is determined by thecon- 'ditions under which the cathode is to run.. In

certain cases it is necessary to add a further exelectron dischargedevices which comprises pro- -viding the surface of the cathode corewith a rare earth metals,- the resulting ions being carthermionicallyactive earth metal oxide, evacu- 2. The process of manufacturing'acathode for electron discharge devices which comprises producing acoating of alkaline earth oxideson the cathode core, preparing afreaction mixture of said metal-metal oxide comlthrough the vapor ofbarium oxide, barium peroxide and aluminum, heating said mixture toreaction temperature to produce a product of metallic barium andaluminum' oxide, heating said' product in the presence of said cathodeto distill the metallic barium therefrom,v passing an ionizing currentsaid distilled, barium whereby the barium is drawn to the oxidesurfaceof said cathode, and thereafter heating said cathode to intermixsaid barium with the oxides at the surface of said cathode. V

. "LESLIE HERBERT BEDFORD.

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